1. Field of the Invention
The present invention relates to a GaAs single crystal substrate and an epitaxial wafer using the same. More specifically, the present invention relates to a GaAs (gallium arsenide) single crystal substrate used for an integrated circuit and a microwave element, and to an epitaxial wafer using the same.
2. Description of the Background Art
Semi-insulating GaAs crystal has been conventionally fabricated through a manufacturing method such as Liquid Encapsulated Czochralski method (LEC method) and Vertical Bridgeman method (VB method).
It has been possible to control dislocation density of the GaAs single crystal substrate within the range of 1000 to 100000 cm.sup.-2, by adjusting temperature gradient and cooling rate at the time of growth.
Carbon concentration and EL2 concentration of the GaAs single crystal substrate have been controlled by adjusting impurity concentration of a solution, ratio of Ga with respect to As and thermal hysterisis after solidification. Generally, two concentrations, that is, the carbon concentration and the EL2 concentration are considered as factors controlling resistivity. With the carbon concentration being in the range of 0.9 to 10.0.times.10.sup.15 cm.sup.-3 and EL2 concentration being 12.0 to 16.0.times.10.sup.15 cm.sup.-3, a substrate having the resistivity of about 0.1 to about 2.0.times.10.sup.8 .OMEGA..multidot.cm has been manufactured.
Reference 1 (T. Kawase et al. Proc. Of the 9th Conf. on Semiconducting and Insulating Materials, Toulouse, France (1996) 275-278) discloses an example of a GaAs single crystal substrate having low dislocation density fabricated through the VB method. The GaAs single crystal substrate has carbon density or concentration of 7 to 8.times.10.sup.15 cm.sup.-3, EL2 concentration after heat treatment of 1.3.times.10.sup.16 cm.sup.-3, resistivity of 3.5 to 6.5.times.10.sup.7 .OMEGA..multidot.cm, mean dislocation density in plane of 1000 to 2000 cm.sup.-2, and mean residual strain measured by photoelastic analysis of 0.2 to 0.3.times.10.sup.-5.
The conventional GaAs single crystal substrate has been expected to be used as a substrate material of electronic devices requiring high speed operation and low power consumption.
Among substrates used for electronic devices, a substrate having an epitaxial thin film layer grown thereon to provide a device operation layer is referred to as an "epitaxial wafer." In manufacturing the epitaxial wafer, when the epitaxial thin film layer is grown, it is necessary to heat the substrate to a temperature called growth temperature, and to bring the surface of the substrate to be in contact with Ga, As and a small amount of impurity called a dopant, in a liquid or gas phase. The epitaxial wafer having a plurality of epitaxial layers stacked on the substrate is subjected to surface etching, deposition of metal for electrodes, and processed into chips, whereby electronic devices are formed. A fundamental characteristic of a thus formed electronic device is amplification of a voltage input signal. Particularly, an electronic device used for satellite communication or the like is required to have a high output and operability at high voltage. Therefore, there has been greater need for a GaAs single crystal substrate having a superior breakdown withstanding characteristic.
The conventional single crystal substrate, however, generally has low carbon concentration and therefore it cannot withstand thermal stress-strain when the temperature is increased to form the actual epitaxial layer, so that steps, which are referred to as slips, result on the substrate surface. When devices are fabricated on the steps, desired device characteristics cannot be attained, and hence production yield is significantly degraded.
Further, the substrate has low resistivity and a number of deep levels such as EL2 within an energy band, and therefore only devices having a low breakdown withstanding characteristic can be provided.